The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Recently, increasing oil prices have caused vehicle manufactures all over the world to rush into infinite competition. Particularly in the case of engines, manufactures have been pursuing efforts to reduce the weight and improve fuel efficiency of vehicles by reducing engine size, etc.
As a result, research into reduction of weight and enhancement of fuel efficiency through downsizing has been conducted for an engine, and research for simultaneously securing operability and fuel efficiency competitiveness through multiple speed stages has been conducted for an automatic transmission.
However, in the automatic transmission, as the number of speed stages increases, the number of internal components also increases, and as a result, the automatic transmission may be difficult to mount, the manufacturing cost and weight may be increased, and power transmission efficiency may be deteriorated.
Accordingly, development of a planetary gear train which may bring about maximum efficiency with a small number of components is important in order to increase a fuel efficiency enhancement effect through the multiple speed stages.
In this aspect, in recent years, 8-speed automatic transmission tends to be implemented, and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
A conventional automatic transmission of eight or more speed stages typically includes three or four planetary gear sets and five or seven control elements (frictional elements), and thus mountability may be deteriorated due to increase of volume and weight.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been attempted. However, such an arrangement is not widely applicable, and using the dog clutches may deteriorate shift-feel.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.